Expose a Sheet object on Reader::XLSX::nextSheet()

Added Sheet class for the XLSX reader that exposes basic sheet info, such as name or ID.
When retrieving the sheet data XML, added extra XML parsing to retrieve sheet data.
Added test

src/Spout/Reader/Internal/XLSX/Worksheet.php

@@ -10,18 +10,23 @@ namespace Box\Spout\Reader\Internal\XLSX;
  */
 class Worksheet
 {
+    /** @var \Box\Spout\Reader\Sheet The "external" sheet */
+    protected $externalSheet;
+
     /** @var int Worksheet number, based on the order of appareance in [Content_Types].xml (zero-based) */
     protected $worksheetNumber;
 
     /** @var string Path of the XML file containing the worksheet data */
     protected $dataXmlFilePath;
 
-    /**
+    /**\
+     * @param \Box\Spout\Reader\Sheet $externalSheet The associated "external" sheet
      * @param int $worksheetNumber Worksheet number, based on the order of appareance in [Content_Types].xml (zero-based)
      * @param string $dataXmlFilePath Path of the XML file containing the worksheet data
      */
-    public function __construct($worksheetNumber, $dataXmlFilePath)
+    public function __construct($externalSheet, $worksheetNumber, $dataXmlFilePath)
     {
+        $this->externalSheet = $externalSheet;
         $this->worksheetNumber = $worksheetNumber;
         $this->dataXmlFilePath = $dataXmlFilePath;
     }
@@ -34,6 +39,14 @@ class Worksheet
         return ltrim($this->dataXmlFilePath, '/');
     }
 
+    /**
+     * @return \Box\Spout\Reader\Sheet The "external" sheet
+     */
+    public function getExternalSheet()
+    {
+        return $this->externalSheet;
+    }
+
     /**
      * @return int
      */